Infrared radiation consists of electromagnetic waves in the wavelength region from 0.75 μm to 1000 μm: lying between visible light and microwave light. An infrared detector is a photodetector that reacts to infrared radiation.
Measurements of thermal radiation from ambient-temperature and higher-temperature objects are utilized in a variety of settings, including but not limited to surveillance, non-destructive testing and defense applications.
In the prior art, these measurements have been obtained using cryogenically-cooled quantum detector materials such as InSb or HgCdTe and/or thermal detectors such as thermopiles, pyroelectrics, or bolometers. Although the quantum detectors have sufficient sensitivities to be able to measure thermal radiation with low noise-equivalent temperature difference (hereinafter “NETD”), the need for cryogens or cryo-coolers and the accompanying vacuum jacket for low-noise operations limit the settings in which a quantum detector can be used.
Additionally, the thermal cycling and eventual ice build-up inside the cryostat leads to poor long-term stability and responsivity. Routine maintenance of the system is cumbersome. Instruments utilizing traditional thermal detectors also suffer from low signal-to-noise ratios resulting from the low-power responsivities of the thermal detectors, which lead to high NETD.
Additionally, accuracy of traditional thermal detectors has been limited by drift of the measured signal when there are changes in the surrounding ambient temperatures. For collection optics, these traditional detectors also require the use of either reflective mirrors or refractive lenses which transmit in the mid-infrared and long-infrared wavelengths, such as those constructed using Si, Ge or ZnSe.
In the past, the use of the SWIR wavelength region for thermal imaging has been limited due to the poor signal-to-noise performances of the traditional thermal detectors. The most commonly utilized thermal detector in the SWIR wavelength region is the photoconductive PbS which requires operation with dry ice at 193 K for optimum operation. However, photoconductive PbS was found to be limited by its high noise-equivalent power, poor long-term stability of response, and possible signal nonlinearity. Other SWIR detectors, such as photoconductive HgCdTe (MCT), PbSe and InAs detectors have also been used, but with limited success.
It is desirable to have a highly responsive apparatus and process for measuring thermal radiation from ambient temperature objects in a variety of settings, without the need for cryogens or cryo-coolers and the accompanying vacuum jacket for low-noise operations, which limits the utility of such devices and methods; the present invention does not require cryogens or cryo-coolers with accompanying vacuum jackets.
It is further desirable to have a device and process that utilizes conventional glass optics. Using conventional glass optics will result in better imaging performance at lower cost and permit the detection of both SWIR and visible light through shared conventional glass optics; the present invention can use conventional glass optics.